Stereo-centre

Asymmetric hydrogenation of enamido (3-keto esters was carried out in the presence of both Rh(I)- and Ru(II)-chiral phosphine complexes as catalysts[1]. This is the efficient method to prepare statin analogues. The process independently induces two stereo centres in a molecule in a simple manner. 

Stereoisomerism in compounds with two stereo centres diastereomers and meso structure In compounds whose stereoisomerism is due to tetrahedral stereocentres, the total number of stereoisomers will not exceed 2", where n is the number of tetrahedral stereocentres. For example, in 2,3,4-trihydroxybutanal, there are two chiral carbons. The chiral centres are at C-2 and C-3. Therefore, the maximum number of possible isomers will be 2 = 4. All four stereoisomers of 2,3,4-trihydroxybutanal (A-D) are optically active, and among them there are two enantiomeric pairs, A and B, and C and D, as shown in the structures below. 

In order to avoid stereo centres and to increase the cytotoxicity, a novel class of substituted titanocene dichloride derivatives, the so-called benzyl-substituted... 

Benzyl-substituted titanocenes do not have stereo centres and therefore stereoisomers do not exist, unlike their ansa analogues. In terms of in vivo and in vitro cell testing, this is advantageous. Previously, the presence of unseparated stereoisomers means that the issue of whether the compounds cytotoxicities are related to specific isomers was not addressed. This is not of concern in the achiral benzyl-substituted titanocenes presented here. 

In summary, this novel approach to substituted titanocene dichlorides delivered compounds with a cytotoxic activity in the range of 240-32 pM, which represents an up to 60-fold increase in cytotoxicity compared with the unsubstituted Cp2TiCl2. The major advantage of diaryhnethyl-functionalised titanocenes is the absence of stereo centres, and with further improvements, there are potential candidates for pre-clinical studies with potential for clinical trials. 

The central point of chirality in a molecule is known as the stereo-centre and diastereoisomers occur when there is more than one stereo-centre in a molecule. Stereo-centres are given the absolute configurations of Rectus (R) and Sinister (S). Diastereoisomers that differ in absolute configuration at the stereo-centres are called epimers. Diastereoisomers differ in conformation so it is actually possible to purify these mixtures by normal phase and reversed phase HPLC. However, better separation factors are often obtained using chiral stationary phases. 

The first new stereocentre is thus taken care of. Next another carbonyl is revealed 197 by ozonolysis of the double bond. We are now in a position to introduce with control the second stereo-centre using the one we have just made ... 

Asymmetric synthesis for quaternary stereo centres Chiral amine synthesis (N-centered chemistry)... 

---